Powder metal with solid lubricant and powder metal scroll compressor made therefrom

ABSTRACT

A powder metal formulation includes a solid lubricant and is particularly useful for the production of powder metal scroll compressors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This claims the benefit of U.S. Provisional Patent Application No.61/599,042 filed Feb. 15, 2012 and U.S. Provisional Patent ApplicationNo. 61/720,226 filed Oct. 30, 2012. The contents of both of theseapplications are incorporated by reference as if set forth in theirentirety herein for all purposes.

BACKGROUND

This disclosure relates to powder metal formulations including solidlubricants and further relates to powder metal parts, such as scrollcompressors, made using these powder metal formulations.

Scroll compressors are typically used for compressing gases or arefrigerant. In such a scroll compressor, two parts are situated so asto have interleaving and complimentary scroll portions. These scrollportions may be shaped as involutes, spirals, or other such curves.

During operation of the scroll compressor, one of the scroll portions isgyrated relative to the other scroll portion. This movement of thescroll portions relative to one another causes the points of contactbetween the two scroll portions to vary. These changing points ofcontact between the scrolls, when made over a continuous length, canresult in the forced movement and the compression of gases and/orrefrigerants between the two parts.

As noted above, the parts for a scroll compressor can have relativelycomplex geometries (i.e., may have an involute or a spiral shape) andcan present fabrication challenges. Because powder metallurgy is welladapted to handle certain complex geometries and high part volumes,powder metal processes have been explored as one means to make thescroll portion of a scroll compressor or, more ambitiously, all of ascroll compressor.

Powder metal parts may be fabricated in the following manner. A powdermetal starting material is compacted under pressure using a die and toolset to form the loose powder metal into a powder metal compact. Thispowder metal compact has a shape that is relatively close to, butslightly larger than, the shape of the final desired part. This powdermetal compact is then sintered to cause the adjacent powder metalparticles to diffuse into one another and to neck together therebybonding the particles together. This sintering is typically done at justbelow the melting temperature of the powder metal material but, in someinstances, a liquid phase may also be developed during sintering. Incomparison to the initial powder metal compact, the sintered powdermetal forms a much stronger sintered part that might be subjected to anyof a number of finishing processes (e.g., machining, grinding,deburring, and so forth), reworking (e.g., forging or coining), orsimply used as-sintered.

SUMMARY

A powder metal formulation is disclosed which is particularly useful inthe production of powder metal scroll compressors. This powder metalformulation includes a solid lubricant such as, for example, talc orboron nitride which is carried through the powder metal formationprocess such that the solid lubricant is part of the final powder metalscroll compressor. In another example, the solid lubricant is anickel-coated graphite powder. In some forms, the solid lubricant isadmixed with the other constituents of the powder metal material. Thesesolid lubricants remain stable at the elevated processing temperaturesemployed during the sintering of the powder metal and so this solidlubricant remains in and, at least to some degree, available at thesurface of the powder metal part after sintering.

Using this powder metal as a starting material, a scroll compressor canbe made that includes a solid lubricant. Among other things, this solidlubricant helps promote smooth contact between the scrolls with reducedamounts of friction. The solid lubricant is also inert such that it doesnot present any concerns when it is used to compress, for example, arefrigerant.

According to one aspect, a powder metal scroll compressor is provided.The powder metal scroll compressor includes a hub and a scroll adjoinedto one another in which a powder metal forms at least a portion of thepowder metal scroll compressor including the scroll. The powder metalincludes iron powder, carbon in an amount of less than 0.9% by weight ofthe powder metal, and a solid lubricant in the powder metal.

The iron powder and solid lubricant may be admixed with one anotherprior to compaction and sintering of the powder metal scroll compressor.

In one form, the solid lubricant may be 0.25% to 3.0% by weight of thepowder metal and the powder metal may include only iron powder, carbon,the solid lubricant and be substantially free of other constituents.

However, in other forms, the powder metal may contain otherconstituents. For example, the powder metal may further include copperpowder (which may be elemental copper powder) in an amount of less than3.0% by weight of the powder metal. In this form, the iron powder, thecopper powder, and the solid lubricant may be admixed with one anotherprior to compaction and sintering of the powder metal scroll compressorand the powder metal may include only iron powder, carbon, copperpowder, and the solid lubricant and be substantially free of otherconstituents.

Various types of solid lubricants may be suitable for use in the powdermetal. In order to provide the lubricating function in the finalcomponent, the solid lubricant should be capable of surviving thecompaction and sintering process (for example, not burn off at sinteringtemperatures). Thus, one having ordinary skill in the art willappreciate that the solid lubricants being referred to are not typicallubricants, waxes, or binders that are conventionally used to help thecompacted powder metal parts retain their shape or be ejected from thecompaction tooling, as those conventional lubricants, waxes, or bindersare consumed and lost during any initial burn off and/or sinteringoperations. Accordingly, many of the solid lubricants described hereinremain inert and stable in an Fe—C or an Fe—Cu—C system throughprocessing of temperatures up to 1080 degrees Centigrade, for example.

One solid lubricant that may be used is talc (Mg₃Si₄O₁₀(OH)₂). The talcmay have a nominal 15 to 25 micron mean particle size (d50).

Another solid lubricant that may be used is hexagonal boron nitride(BN). The hexagonal boron nitride may have a nominal 5 to 30 micron meanparticle size (d50).

The solid lubricant may be provided in the form of a nickel-coatedgraphite powder. In this instance, the carbon may be present in anamount of less than 0.9% by weight of the powder metal material,exclusive of the graphite of the nickel-coated graphite powder (as thisgraphite powder does not significantly contribute to the carbon contentin the iron). A nickel coating of the nickel-coated graphite powder maysubstantially surround the graphite to protect the graphite duringsintering of the powder metal scroll compressor and to prevent thegraphite from combining with the iron powder. A nickel content of thenickel-coated graphite powder may be in a range of 55 to 80 wt % withthe remainder being graphite. A total amount of graphite in the powdermetal scroll compressor may be in the range of 0.5 to 5.0 wt %, or morenarrowly 1.0 to 3.0 wt %, exclusive of the carbon in an amount of lessthan 0.9% by weight of the powder metal material. The nickel-coatedgraphite powder may have an average particle size of approximately 100microns.

According to another aspect, a powder metal is provided, such as apowder metal that may be used for a powder metal scroll compressor ofthe type described above. The powder metal includes iron powder, carbonin an amount of less than 0.9% by weight of the powder metal material,and a solid lubricant. The iron powder and solid lubricant are admixedwith one another.

In one form of the powder metal, the powder metal may include ironpowder, carbon, the solid lubricant and be substantially free of otherconstituents.

In another form, the powder metal may further include a copper powder(such as an elemental copper powder) in an amount of less than 3.0% byweight of the powder metal. The iron powder, the copper powder, and thesolid lubricant may be admixed with one another, and the powder metalmay include iron powder, carbon, copper powder, and the solid lubricantand be substantially free of other constituents.

The solid lubricant may be 0.25% to 3.0% by weight of the powder metal.For the reasons identified above relating to the retention of the solidlubricant in the final part, the solid lubricant may remain inert andstable in an Fe—C or an Fe—Cu—C system through processing oftemperatures up to 1080 degrees Centigrade.

In one form of the powder metal, the solid lubricant may be talc(Mg₃Si₄O₁₀(OH)₂). The talc may have a nominal 15 to 25 micron meanparticle size (d50).

In another form, the solid lubricant may be hexagonal boron nitride(BN). The hexagonal boron nitride may have a nominal 5 to 30 micron meanparticle size (d50).

In still another form, the solid lubricant may be a nickel-coatedgraphite powder and in which the carbon in an amount of less than 0.9%by weight of the powder metal material is exclusive of the graphite ofthe nickel-coated graphite powder.

A nickel coating of the nickel-coated graphite powder may substantiallysurround the graphite to protect the graphite during sintering of thepowder metal scroll compressor and to prevent the graphite fromcombining with the iron powder. A nickel content of the nickel-coatedgraphite powder may be in a range of 55 to 80 wt % with the remainderbeing graphite. A total amount of graphite in the powder metal scrollcompressor may be in the range of 0.5 to 5.0 wt %, or more narrowly 1.0to 3.0 wt %, exclusive of the carbon in an amount of less than 0.9% byweight of the powder metal material. The nickel-coated graphite powdermay be an average particle size of approximately 100 microns.

A part may be made using any of the powder metal formulations describedherein by compacting and sintering the powder metal to form the part.The solid lubricant is retained throughout the process and is dispersedthroughout the part including the surface of the part. It may beparticularly advantageous when this surface of the part is a bearingsurface such that the solid lubricant can serve as a lubricant on thissurface.

These and still other advantages of the invention will be apparent fromthe detailed description and drawings. What follows is merely adescription of some preferred embodiments of the present invention. Toassess the full scope of the invention, the claims should be looked toas these preferred embodiments are not intended to be the onlyembodiments within the scope of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top perspective view of a one-piece scroll compressorshowing the hub side.

FIG. 2 is a bottom perspective view of the scroll compressor of FIG. 1showing the scroll side.

FIG. 3 is a top plan view of the scroll compressor of FIG. 1.

FIG. 4 is a cross-sectional side view of the scroll compressor takenalong line 4-4 of FIG. 3.

DETAILED DESCRIPTION

Referring first to FIGS. 1 through 4, a one-piece scroll compressor 100is shown. The powder metal scroll compressor 100 can be produced from asingle powder metal compact using the powder metal processes accordingto the method described in PCT International Publication No. WO2010/135232 later filed as a U.S. national phase application having Ser.No. 13/320,867 and published as US 2012/0118104, which is incorporatedby reference as if set forth in its entirety herein.

To be clear, in the instant disclosure, the powder metal formulationcould be used to separately make one or more portions of the part or theentirety of the part using, for example, the methods described in PCTInternational Publication No. WO 2010/135232 and U.S. Patent ApplicationPublication No. US 2012/0118104. However, the details of the structureand the processes used to fabricate the scroll compressor should not beso limited to only the structures and methods explicitly listed.Accordingly, the below described scroll compressor is intended to beillustrative, but not limiting.

The scroll compressor 100 is a powder metal part which is formed bycompression along an axis of compaction A-A. The scroll compressor 100includes a flange 102, a hub 104, and a scroll 106. The flange 102 has atop face 108 and a bottom face 110 which extend in a directionperpendicular to the axis A-A and which are essentially planar andparallel to one another. Two mounting slots 112 are formed around anouter periphery of the flange 102 for mounting the scroll compressor 100to another item in a refrigeration assembly or the like.

The hub 104 axially extends from the top face 108 of the flange 102. Thehub 104 is generally cylindrically-shaped and has a radially outwardfacing surface 114, a radially inward facing surface 116, and a topaxial face 118. Both the radially outward facing surface 114 and theradially inward facing surface 116 may have a slight taper as theyextend away from the top face 108 of the flange 102 towards the topaxial face 118. By having a taper, the scroll compressor 100 can be moreeasily separated from the tool members during the ejection process.

The scroll 106 extends axially from the bottom face 110 of the flange102. The scroll 106 is a spiraling wall that spirals relative to theaxis A-A. As such, the scroll 106 includes an inner wall end 120 and anouter wall end 122 with a generally radially outward facing surface 124and a generally radially inward facing surface 126 extending between theends 120 and 122. These surfaces 124 and 126 run generally parallel toone another as they spiral away from the axis A-A, creating a spiralingwall of uniform thickness. A bottom axial face 128 of the scroll 106 isalso spiral-shaped. Again, the generally radially outward facing surface124 and the generally radially inward facing surface 126 may have ataper to ease the ejection process from the tool and die set duringcompaction of the powder metal.

In the form shown, the spiral is similar to an Archimedean spiral,meaning that if a radial line is drawn relative to the axis A-A, achannel 130 formed between the generally radially outward and inwardfacing surfaces 124 and 126 is also of substantially constant widthregardless of the distance from the axis A-A. However, other involutegeometries might be used and nothing should limit the scroll compressorgeometry to that which is illustrated in FIGS. 1 through 4.

A part having this geometry could not be easily formed as a unitarypowder metal compact by a conventional powder metal compaction process.Typically, top features, such as the hub 104 are formed by transferringpowder metal within the die cavity by a powder transfer motion of thelower tool members. As the powder is transferred, the powder fill tofinal part ratio along the vertical columns of the part must beapproximately 2:1 to provide a part that is relatively uniformly denseafter the compaction process.

A comparison of a horizontal cross section through the hub 104 and ahorizontal cross section through the scroll 106 would reveal that thereare areas of powder metal in the hub 104 which are not found in thescroll 106 and areas of powder metal in the scroll 106 that are notfound in the hub 104. Thus, conventional tool and die sets are incapableof performing a powder transfer motion that provides an acceptablepowder fill to final part ratio over a component having this finalgeometry. Instead, the hub and scroll sections are conventionallyseparately compacted and then joined afterwards. However, PCTInternational Publication No. WO 2010/135232 and U.S. Patent ApplicationPublication No. US 2012/0118104 describe ways of fabricating a unitarypart. Parts made using both the conventional and improved methods arecontemplated as being within the scope of this invention.

Turning now to the powder metal formulation, the powder metal used tomake this powder metal scroll compressor includes iron powder (eitherelemental or prealloyed iron), carbon in an amount less than 0.9 wt % ofthe powder metal, and solid lubricant in an amount between 0.25 wt % and3.0 wt % of the powder metal. Other elemental additions could also beincluded such as, for example, copper (Cu) and nickel (Ni). However, thepowder metal formulation is relatively simplistic in that it does notrequire more than these listed constituents, but may include traceamounts of other elements that do not substantially affect theproperties of the powder metal.

Mixing of the various constituent powders may be performed usingconventional means such as v-blenders or double cone mixers. The variousconstituent powders can be admixed together along with pressinglubricants (e.g., lithium stearate, Licowax, etc.) in addition to thesolid lubricant.

In one form, the solid lubricant for this powder metal formulation maybe talc, which is also known as hydrated magnesium silicate and has thechemical formula of Mg₃Si₄O₁₀(OH)₂. When used as a solid lubricant, someamount of quartz impurity may exist within the talc. When talc is usedas the solid lubricant, the talc may be preferably provided in a powderform having a nominal 15 to 25 micron mean particle size (d50).

In another form, the solid lubricant for this powder formulation may bea hexagonal boron nitride (BN). When used as a solid lubricant, theboron nitride may be preferably provided in a powder form having anominal 5 to 30 micron mean particle size (d50).

Another approach to provide an in-place solid lubricant in a powdermetal component is to use nickel-coated graphite powder as the solidlubricant. The nickel coating protects the graphite during sintering andprevents the graphite from combining with the iron powder. In thefinished product, the coating protects and preserves the graphite untilrupture of the nickel coating during use of the component (e.g., rupturedue to wear on surfaces), to release the graphite lubricant. Thenickel-coated graphite has nickel content ranging from 55 to 80 wt %with the remainder being graphite. The nickel-coated graphite powder mayhave an average particle size of approximately 100 microns. Graphitesize may be coarse (Tyler mesh size −120/+230 at 88-98% or a range of115 to 65 microns) to fine (Tyler mesh size −120/+270 at >85% and−270/+325<15% or a range of 115 to 43 microns) with both having a smallamount of more coarse and finer particle sizes (<5 wt %). The solidlubricant is added to the mix to provide a graphite level of 0.5 wt % to5 wt %, although a graphite range of 1 wt % to 3 wt % is believed to betypical for most applications. This graphite is exclusive of the carboncontent in the powder metal which is used to alter the metallurgicalproperties of the iron powder.

Notably, this powder metal formulation incorporates the powder for thesolid lubricant as an admixed constituent in the powder metal. Theadmixed solid lubricant is inert in Fe—C, Fe—Cu—C and other powder metalmixtures processed at temperatures around 1180 degrees Centigrade. Thismeans that even after the powder metal has been compacted and sinteredinto a final part, all or a substantial portion of the solid lubricantremains present.

The solid lubricant assists in reducing frictional heat andspalling/galling in a system with reciprocal motion under a mechanicalload such as that in which a scroll compressor is used (and, inparticular, in the scroll section of the scroll compressor). The solidlubricant is resistant to adhesion and does not create significantresistance to motion of the scroll compressor. The inclusion of thesolid lubricant may be best implemented in powder metal parts that haveferritic/pearlitic microstructures and its inclusion can also be usedfor improving machinability.

In one specific embodiment that is particularly advantageous for powdermetal scroll compressors, the powder metal formulation can be formulatedto have less than 0.9 wt % carbon, less than 3.0% copper, and between0.25 to 3.0 wt % of solid lubricant with the remainder of the powderbeing elemental iron with no other substantial additions. Again, this isa relatively simple powder formula that does not contain a large numberof alloying elements.

Powder metal formulations such as those described above can be preparedand then processed into a sintered powder metal part by compacting thepowder into a powder metal compact and then sintering the powder. It iscontemplated that such parts might be compacted as unitary bodies ormight be formed from separately compacted components that aresubsequently joined together to form a single final part. However, it iscontemplated that any section of a part made from various joinedsections may have the powder metal containing the solid lubricant inthose sections in which the solid lubricant will be most desirable. Forexample, the scroll section of a scroll compressor may be made using thepowder described above, while the other section to which the scrollsection is joined may be made of a powder metal material that does notinclude the solid lubricant. Of course, nothing excludes both sectionsof a multi-portion component from being made of a powder containing thesolid lubricant even if they are joined.

Overall, this process allows for conventional compaction processes inrigid dies and eliminates the subsequent infiltration of a solidlubricant into the porous sintered body after sintering.

It should be appreciated that various other modifications and variationsto the preferred embodiments can be made within the spirit and scope ofthe invention. Therefore, the invention should not be limited to thedescribed embodiments. To ascertain the full scope of the invention, thefollowing claims should be referenced.

What is claimed is:
 1. A powder metal scroll compressor comprising: ahub and a scroll adjoined to one another; a powder metal forming atleast a portion of the powder metal scroll compressor including thescroll, the powder metal including iron powder, carbon in an amount ofless than 0.9% by weight of the powder metal, and a solid lubricant inthe powder metal.
 2. The powder metal scroll compressor of claim 1,wherein the iron powder and solid lubricant are admixed with one anotherprior to compaction and sintering of the powder metal scroll compressor.3. The powder metal scroll compressor of claim 1, wherein the solidlubricant is 0.25% to 3.0% by weight of the powder metal and the powdermetal includes iron powder, carbon, the solid lubricant and issubstantially free of other constituents.
 4. The powder metal scrollcompressor of claim 1, wherein the powder metal further includes copperpowder in an amount of less than 3.0% by weight of the powder metal. 5.The powder metal scroll compressor of claim 4, wherein the iron powder,the copper powder, and the solid lubricant are admixed with one anotherprior to compaction and sintering of the powder metal scroll compressorand the powder metal includes iron powder, carbon, copper powder, andthe solid lubricant and is substantially free of other constituents. 6.The powder metal scroll compressor of claim 4, wherein the copper powderis elemental copper powder.
 7. The powder metal scroll compressor ofclaim 1, wherein the solid lubricant is talc (Mg₃Si₄O₁₀(OH)₂).
 8. Thepowder metal scroll compressor of claim 7, wherein the talc has anominal 15 to 25 micron mean particle size (d50).
 9. The powder metalscroll compressor of claim 1, wherein the solid lubricant is hexagonalboron nitride (BN).
 10. The powder metal scroll compressor of claim 9,wherein the hexagonal boron nitride has a nominal 5 to 30 micron meanparticle size (d50).
 11. The powder metal scroll compressor of claim 1,wherein the solid lubricant remains inert and stable in an Fe—C or anFe—Cu—C system through processing of temperatures up to 1080 degreesCentigrade.
 12. The powder metal scroll compressor of claim 1, whereinthe solid lubricant is a nickel-coated graphite powder and in which thecarbon in an amount of less than 0.9% by weight of the powder metalmaterial is exclusive of the graphite of the nickel-coated graphitepowder.
 13. The powder metal scroll compressor of claim 12, wherein anickel coating of the nickel-coated graphite powder substantiallysurrounds the graphite to protect the graphite during sintering of thepowder metal scroll compressor and to prevent the graphite fromcombining with the iron powder.
 14. The powder metal scroll compressorof claim 12, wherein a nickel content of the nickel-coated graphitepowder is in a range of 55 to 80 wt % with the remainder being graphite.15. The powder metal scroll compressor of claim 12, wherein a totalamount of graphite in the powder metal scroll compressor is in the rangeof 0.5 to 5.0 wt % exclusive of the carbon in an amount of less than0.9% by weight of the powder metal material.
 16. The powder metal scrollcompressor of claim 12, wherein a total amount of graphite in the powdermetal scroll compressor is in the range of 1.0 to 3.0 wt % exclusive ofthe carbon in an amount of less than 0.9% by weight of the powder metalmaterial.
 17. The powder metal scroll compressor of claim 12, whereinthe nickel-coated graphite powder has an average particle size ofapproximately 100 microns.
 18. A powder metal comprising: iron powder;carbon in an amount of less than 0.9% by weight of the powder metalmaterial; and a solid lubricant; in which the iron powder and solidlubricant are admixed with one another.
 19. The powder metal of claim18, wherein the solid lubricant is 0.25% to 3.0% by weight of the powdermetal.
 20. The powder metal of claim 18, wherein the powder metalfurther includes copper powder in an amount of less than 3.0% by weightof the powder metal.
 21. The powder metal of claim 20, wherein the ironpowder, the copper powder, and the solid lubricant are admixed with oneanother and the powder metal includes iron powder, carbon, copperpowder, and the solid lubricant and is substantially free of otherconstituents.
 22. The powder metal of claim 20, wherein the copperpowder is elemental copper powder.
 23. The powder metal of claim 20,wherein the solid lubricant is talc (Mg₃Si₄O₁₀(OH)₂).
 24. The powdermetal of claim 23, wherein the talc has a nominal 15 to 25 micron meanparticle size (d50).
 25. The powder metal of claim 18, wherein the solidlubricant is hexagonal boron nitride (BN).
 26. The powder metal of claim25, wherein the hexagonal boron nitride has a nominal 5 to 30 micronmean particle size (d50).
 27. The powder metal of claim 18, wherein thesolid lubricant remains inert and stable in an Fe—C or an Fe—Cu—C systemthrough processing of temperatures up to 1080 degrees Centigrade. 28.The powder metal of claim 18, wherein the powder metal includes ironpowder, carbon, the solid lubricant and is substantially free of otherconstituents.
 29. The powder metal of claim 18, wherein the solidlubricant is a nickel-coated graphite powder and in which the carbon inan amount of less than 0.9% by weight of the powder metal material isexclusive of the graphite of the nickel-coated graphite powder.
 30. Thepowder metal of claim 29, wherein a nickel coating of the nickel-coatedgraphite powder substantially surrounds the graphite to protect thegraphite during sintering of the powder metal scroll compressor and toprevent the graphite from combining with the iron powder.
 31. The powdermetal of claim 29, wherein a nickel content of the nickel-coatedgraphite powder is in a range of 55 to 80 wt % with the remainder beinggraphite.
 32. The powder metal of claim 29, wherein a total amount ofgraphite in the powder metal scroll compressor is in the range of 0.5 to5.0 wt % exclusive of the carbon in an amount of less than 0.9% byweight of the powder metal material.
 33. The powder metal of claim 29,wherein a total amount of graphite in the powder metal scroll compressoris in the range of 1.0 to 3.0 wt % exclusive of the carbon in an amountof less than 0.9% by weight of the powder metal material.
 34. The powdermetal of claim 29, wherein the nickel-coated graphite powder has anaverage particle size of approximately 100 microns.
 35. A part madeusing the powder metal of claim 18, wherein the powder metal iscompacted and sintered to form the part and the solid lubricant isretained throughout the process and is dispersed throughout the partincluding the surface of the part.